I've been there before, I used a straight edge to align myself with the wall that turned perpendicular. I marked the top and the bottom. Stuck a piece of masking tape on the top mark, stretched it straight, down to the floor. Aligned with the bottom mark and rubbed the tape down firmly. Paint to the tape, when finished, peel it back over itself to break the edge clean.
I've also seen people set half columns at the end of hallways. A half on each side. Painted different wall colors from there.
Just a suggestion, Welcome to the site. Don't hesitate to post again.
Tom in KY, yep, I paint too.